Alzheimer's disease is an age-associated neurodegenerative disorder of the brain. The disorder is characterized histopathologically by the formation and accumulation of neurofibrillary tangles and neuritic plaques in the brain. In particular, pathological changes associated with the disease extensively affect neurons in the olfactory bulb and its connected brain structures. Degeneration with loss of neurons has been observed in the hippocampal formation, amygdaloid nuclei, nucleus basalis of Meynert, locus ceruleus, and the brainstem raphe nuclei, all of which project to the olfactory bulb. These degenerative changes result in the loss of memory and cognitive function. In addition, there is a major loss of cortical and hippocampal choline acetyltransferase activity and degeneration of basal forebrain cholinergic neurons. The loss of odor detection in Alzheimer's patents has been attributed to necrosis of olfactory epithelium, olfactory bulbs and tracts and the prepyriform cortex.
At present, there is no treatment for Alzheimer's disease which effectively prevents or retards the progressive neurodegeneration of the brain and the loss of smell and cognitive decline associated with the illness. Neurotrophic and neuritogenic factors, such as nerve growth factor (NGF) and gangliosides, have demonstrated therapeutic effects in animal models and cell cultures which indicate these substances may be of benefit to patients afflicted with Alzheimer's disease. See Frey, W. H., II and T. A. Ala, Progress in Clinical Neuroscience 1:287-303 (1988).
Neurotrophic and neuritogenic factors are agents that affect the survival and differentiation of neurons in the peripheral and central nervous systems. These growth promoting factors are signaling substances that are synthesized in tissues in response to neurons capable of responding to the factor. They bind to receptors on the surface of nerve cells to promote neuron survival and in some cases are incorporated into nerve cell membranes. Studies further indicate that nerve growth factor (NGF), a class of polypeptide signaling substances, may be capable of improving cholinergic functioning which would prevent injury-induced degeneration of basal forebrain cholinergic neurons and improve cognitive functioning. Nerve growth factor (NGF) is known to bind to receptors on axon terminals, and can be internalized and retrogradely transported to the cell body of neurons. See M. Seiler, Brain Res. 300:33-39 (1984). Other naturally-occurring nerve growth promoting factors include gangliosides, phosphatidylserine (PS), brain-derived neurotrophic factor, fibroblast growth factor, insulin, insulin-like growth factors, ciliary neurotrophic factor and glia-derived nexin.
Testing the effectiveness of potentially therapeutic agents against brain disease in animal toxicity studies and human trials has been hindered, however, by the inability of existing procedures to readily deliver adequate levels of the agent to affected areas of the brain over an extended period of time.
Some experimental therapeutic agents used in the treatment of Alzheimer's disease, such as GM-1 ganglioside, can be administered to the brain through the bloodstream because of their ability to traverse the blood-brain barrier. However, it is not clear that effective levels of the ganglioside reach the affected areas of the brain.
Other potentially therapeutic agents, such as nerve growth factor (NGF), are unable to cross the blood-brain barrier and must be administered to the brain by other means. One such method of delivery is by an intracerebroventricular pump. Use of such a pump, however, necessitates invasive surgery which can entail a variety of medically-related complications. Furthermore, administration of medication by pump does not facilitate selective delivery of medication solely to those areas of the brain affected by disease. Consequently, healthy areas of the brain may be adversely affected by the neurologic agent while some diseased areas may not receive a high enough level for adequate treatment or testing of a drug.
An effective method of therapeutic intervention is needed to prevent and effectively treat brain diseases such as Alzheimer's disease, Parkinson's disease, nerve damage from cerebrovascular disorders such as stroke, and ordinary aging. Testing the potential of various neurologic agents is an important aspect of developing treatments for neurodegenerative diseases. Since existing methods of testing possible therapeutic agents and treating brain disorders are of limited benefit, a goal of the present invention is to develop a procedure to effectively deliver neurologic agents to the brain. A particular goal of the invention is to develop a method of delivering neurologic substances to the brain to augment the level of activity against brain diseases by naturally-occurring substances. A further goal is to develop a means of selective delivery of a neurologic agent only to areas of the brain which are damaged by a brain disorder. Still another objective is to develop a composition that can cause absorption of the neurologic agent into olfactory neurons and along the olfactory neural pathway to damaged neurons in the brain. Another goal is to provide prophylactic treatment of neurodegenerative diseases and to treat and/or prevent associated loss of smell.